1. Field of the Invention
The invention relates generally to pipes and tubing used in the oilfield. Specifically, the invention relates to an improved method for removing mineral scale from pipes and tubing.
2. Background Art
Hydrocarbons (e.g., oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a wellbore that penetrates the hydrocarbon-bearing formation. In order for the hydrocarbons to be produced, that is, travel from the formation to the wellbore, and ultimately to the surface, at rates of flow sufficient to justify their recovery, a sufficiently unimpeded flowpath from the subterranean formation to the wellbore, and then to the surface, must exist or be provided.
Subterranean oil recovery operations may involve the injection of an aqueous solution into the oil formation to help move the oil through the formation and to maintain the pressure in the reservoir as fluids are being removed. The injected aqueous solution, usually surface water (lake or river) or seawater (for operations offshore), generally contains soluble salts such as sulfates and carbonates. These salts may be incompatible with the ions already contained in the oil-containing reservoir. The reservoir fluids may contain high concentrations of certain ions that are encountered at much lower levels in normal surface water, such as strontium, barium, zinc and calcium. Partially soluble inorganic salts, such as barium sulfate (or barite) and calcium carbonate, often precipitate from the production water as conditions affecting solubility, such as temperature and pressure, change within the producing well bores and topsides.
A common reason for a decline in hydrocarbon production is the formation of scale in or on the wellbore, in the near-wellbore area or region of the hydrocarbon-bearing formation matrix, and in other pipes or tubing. Oilfield operations often result in the production of fluid containing saline-waters as well as hydrocarbons. The fluid is transported from the reservoir via pipes and tubing to a separation facility, where the saline-waters are separated from the valuable hydrocarbon liquids and gasses. The saline-waters are then processed and discharged as waste water or re-injected into the reservoir to help maintain reservoir pressure. The saline-waters are often rich in mineral ions such as calcium, barium, strontium and iron anions and bicarbonate, carbonate and sulphate cations. Generally, scale formation occurs from the precipitation of minerals, such as barium sulfate, calcium sulfate, and calcium carbonate, which become affixed to or lodged in the pipe or tubing. When the water (and hence the dissolved minerals) contacts the pipe or tubing wall, the dissolved minerals may begin to precipitate, forming scale. These mineral scales may adhere to pipe walls as layers that reduce the inner bore of the pipe, thereby causing flow restrictions. Not uncommonly, scale may form to such an extent that it may completely choke off a pipe. Oilfield production operations may be compromised by such mineral scale. Therefore, pipes and tubing may be cleaned or replaced to restore production efficiency.
Some mineral scales, such as barium sulphate, are very difficult to remove chemically, from tubing and, as such, the tubing is simply replaced with new tubing. The scaled tubing may be removed for disposal, but the mineral scale that forms presents an environmental hazard. For example, some mineral scales may have the potential to contain naturally occurring radioactive material (NORM). The scale has an associated radioactivity because the radioactive decay daughters of Uranium and Thorium are naturally present in reservoir waters and co-precipitate with barium ions to form a barium sulphate scale that, for example, contains Radium-226 Sulphate. The primary radionuclides contaminating oilfield equipment include Radium-226 (226Ra) and Radium-228 (228Ra), which are formed from the radioactive decay of Uranium-238 (233U) and Thorium-232 (232Th). While 238U and 232Th are found in many underground formations, they are not very soluble in the reservoir fluid. However, the daughter products, 226Ra and 228Ra, are soluble and can migrate as ions into the reservoir fluids to eventually contact the injected water. While these radionuclides do not precipitate directly, they are generally co-precipitated in barium sulfate scale, causing the scale to be mildly radioactive. This NORM poses a hazard to people coming into contact with it through irradiation and through breathing or ingestion of NORM particles. As a result, the NORM scaled tubing has to be handled, transported, and disposed of under carefully controlled conditions, as outlined in legislation, to protect the welfare of employees, the public at large, and the environment.
Common operations used for removing scale from tubing may be slow and inefficient because each tube has to be individually treated if they are radioactive and access to the scaled internal surface of the tubing may be restricted.
When pipes and equipment used in oilfield operations become layered with scale, the encrustation must be removed in a time- and cost-efficient manner. Occasionally, contaminated tubing and equipment is simply removed and replaced with new equipment. When the old equipment is contaminated with NORM, this scale encrusted equipment may not be disposed of easily because of the radioactive nature of the waste. The dissolution of NORM scale and its disposal may be costly and hazardous. In addition, a considerable amount of oilfield tubular goods and other equipment awaiting decontamination is presently sitting in storage facilities. Some equipment, once cleaned, may be reused, while other equipment must be disposed of as scrap. Once removed from the equipment, several options for the disposal of NORM exist, including deep well injection, landfill disposal, and salt cavern injection.
Typical equipment decontamination processes have included both chemical and mechanical efforts, such as milling, high pressure water jetting, sand blasting, cryogenic immersion, and chemical chelants and solvents. Water jetting using pressures in excess of 140 MPa (with and without abrasives) has been the predominant technique used for NORM removal. However, use of high pressure water jetting is generally time consuming, expensive, and may fail to thoroughly treat the contaminated area.
While chemical chelants, such as EDTA (ethylenediaminetctraacetic acid) or DTPA (diethylenetriaminepentaacetic acid), have long been used to remove scale from oilfield equipment, once EDTA becomes saturated with scale metal cations, the spent solvent is generally disposed of, such as by re-injection into the subsurface formation. Further, chemical chelants such as EDTA and DTPA are expensive and require prolonged contact at elevated temperatures to dissolve the scale.
Accordingly, there exists a need for an economically efficient means for removing scale from pipes and tubing with a low risk of exposure to radioactive materials.